For some reason Avios seem to want to be clever and automatically locate your account based on your details rather than just asking you for the number! Same when you convert Tesco points. Sometimes they get it right - you will only know after your first statement is generated and can check your Avios account to see if they have been credited. If not, as above they have probably created a duplicate account for you.

This happened to me but I don't recall it being much of an issue to resolve with Avios - they must surely be used to it!